For some time, refrigerator units for container have been used to cool the inside of containers used for freight transport and the like. Some of these refrigerator units for container are equipped with ventilation units for ventilating the interior of the container. For example, in the case of a container used for transporting fruits and vegetables, it is necessary to provide an appropriate degree of ventilation of the air inside the container in order to keep the fruits and vegetables fresh. A ventilation unit is therefore used to accomplish the ventilation of the interior of the container (see Japanese Laid-Open Patent Publication No. 9-280720).
Meanwhile, there is a need to know the quantity of air that is exchanged by the ventilation units used in container refrigeration units. In the example presented above, since the ventilation affects the freshness of the fruits and vegetables, knowing the quantity of air that has been ventilated is useful for maintaining the freshness of the fruits and vegetables. Also, if the quantity of ventilation is known, a transport company transporting fruits and vegetables can provide a fruit and vegetable owner with a guarantee that an appropriate degree of ventilation is being conducted.
However, it is difficult to know the quantity of air that is ventilated to and from conventional container refrigeration units like that just described.